You can get an effective aerobic workout with both an elliptical machine and a treadmill. In general, you can let your fitness goals determine whether you choose an elliptical machine or a treadmill — or a different piece of exercise equipment.

However, elliptical machines might offer some advantages over treadmills. For example:

Using an elliptical machine can be less stressful on your knees, hips and back than is running on a treadmill. Walking on a treadmill, however, exerts about the same force as using an elliptical machine.

Unlike treadmills, some elliptical machines are equipped with movable upper body handles or poles, similar to ski poles. These allow you to exercise both your arms and your legs.

Most elliptical machines can be pedaled in reverse, which allows you to work your calf and hamstring muscles a bit more than does forward motion.

Using an elliptical machine is generally considered low impact. An elliptical machine might be a good alternative to jogging, whether on a treadmill or outside.

An elliptical trainer shouldn't cause knee pain if you're using it correctly. You may experience knee pain, though, if you have an underlying knee problem, such as degenerative arthritis. With certain knee injuries, using a stationary bike might be a more appropriate option than using an elliptical machine. Talk to your doctor about what exercise is right for you if you have any injuries or health concerns.

And what if you're training for a 5K run or other road race? A treadmill is probably a better tool to prepare you for running events. But even if running is your main aerobic fitness activity, cross-training with an elliptical machine or other low-impact exercise equipment can help keep you fresh and prevent overload injuries.

If you use an elliptical machine, remember to maintain good posture to help ensure the most effective workout. Keep your shoulders back, your head up and your abdominal muscles tight. Look forward, not down at your feet. And don't lean on the handles — let your lower body support your weight.